Backhaul signal compression through spatial-temporal linear prediction

ABSTRACT

The technology in this application compresses multi-antenna complex-valued signals by exploiting both a spatial and a temporal correlation of the signals to remove redundancy within the complex-valued signals and substantially reduce the capacity requirement of backhaul links. At a receiver, the compressed signal is received, and a decompressor decompresses the received signal over space and over time to reconstruct the multiple antenna stream.

TECHNICAL FIELD

The technical field relates to communications, and more particularly, todata compression in order to communicate more information for a givenamount of communication resources.

BACKGROUND

The exponential growth in the demand for wireless data communicationshas put tremendous pressure on cellular network operators to improve thecapacity of their communication networks. To improve the spectralefficiency of these networks, scarce radio resources need to be reusedaggressively in neighboring cells. As a result, inter-cell interferenceis a significant source of signal disturbance, limiting both the servicequality of cell-edge users and overall system throughput.

Coordinated multi-point (CoMP) transmission or reception is onepromising option because of its promise to effectively mitigateinter-cell interference. The idea behind CoMP in the downlink is toconnect multiple remote base-stations via certain backhaul communicationlinks from several adjacent cells to a central processor (CP) to form a“super-cell,” or a CoMP cluster, such that transmission to or receptionfrom multiple user equipments (UEs) within each CoMP cluster can becoordinated by the central processor to reduce or even avoid mutualinterference among UEs. The benefit attainable by the deployment of CoMPdepends on how well that coordination can be performed by the CP.

To enable the central processor to effectively coordinate transmissionand/or reception at multiple cells, signal information must becommunicated between remote base station sites and CP in a timelyfashion. But the amount of information that must be sent to or receivedfrom each remote site can be overwhelming, especially when multipleantennas are deployed at each site. For example, in the Common PublicRadio Interface (CPRI), each real-valued sample of the IQ(complex-valued) backhaul signal is simply quantized independently by afixed number of bits (e.g., 15 bits). It does not exploit any structureof the underlying backhaul signal and is an inefficient way ofrepresenting wireless communication signal. This puts an unnecessarilylarge burden on the capacity of backhaul links. What is needed is aneffective method to compress those multi-antenna signals with bothin-phase (I) and quadrature-phase (Q) components for each antennabranch.

SUMMARY

The technology in this application compresses multi-antennacomplex-valued signals by exploiting both a spatial and a temporalcorrelation of the signals to remove redundancy within thecomplex-valued signals and substantially reduce the capacity requirementof backhaul links. At a receiver, the compressed signal is received, anda decompressor decompresses the received signal over space and over timeto reconstruct the multiple antenna streams.

One aspect of the technology relates to a compression method forcompressing information in signals received at multiple antennas. Thereceived multiple antenna signals are decorrelated over space and overtime to generate a compressed signal, which is transmitted to areceiving node.

In one non-limiting example implementation, the multiple antenna signalsare part of a coordinated multi-point communication, and thetransmitting is from or towards multiple geographically separatedlocations over one or more backhaul communications links. The multipleantenna signals are complex-valued and sampled.

In a preferred example embodiment, the decorrelating includes generatingpredictions of the multiple antenna signals, which in an exampleimplementation includes finite impulse response (FIR) filtering.Associated error signals between the predicted multiple antenna signalsand corresponding ones of the received multiple antenna signals aredetermined to remove time correlation from the received multiple antennasignals, and the error signals are used to generate the compressedsignal. The error signals are transformed using a linear spatialtransformation into spatially-transformed errors to remove correlationin space across different antennas in the received multiple antennasignals. Each of the spatially-transformed errors is then quantized suchthat the compressed signal includes the quantized, spatially-transformederrors.

The spatial linear transform may use fixed, predetermined transformcoefficients corresponding for example to a discrete-cosine transform(DCT), a discrete Fourier transform (DFT), or a discrete wavelettransform (DWT). Alternatively, the spatial linear transform includesadaptively computed transform coefficients, in which case, the adaptivetransform coefficients are sent to the receiving node. One non-limitingexample is a spatial linear transform that includes transformcoefficients corresponding to a Kahunen-Loeve transform (KLT).

The spatially-transformed errors may be quantized using a predeterminedor adaptively selected number of bits. And one or more of thespatially-transformed errors may not quantized if desired. If apredetermined, fixed number of bits is used the same number of bits maybe used to quantize all spatially transformed errors. If an adaptivelycomputed number of bits is used, that adaptively computed number maydepend on corresponding variances of the spatially-transformed errorsover time such that a larger number of bits is allocated for quantizinga spatially-transformed error with a larger corresponding variance, anda smaller number of bits is allocated for quantizing aspatially-transformed error with a smaller corresponding variance. Inone example embodiment, a Breiman-Friedman-Olshen-Store (BFOS) algorithmmay be used to allocate bits for quantizing the spatially-transformederrors. In another example embodiment, bits are allocated according to alogarithm of the variances of the spatially transformed errors forquantizing the spatially-transformed errors. The adaptively selected bitallocations for the spatially-transformed errors may be sent to thereceiving node.

The spatially-transformed errors may be quantized in an exampleembodiment using variable-rate quantizers. Moreover,spatially-transformed errors may be quantized using quantizers withuniform step or cell sizes. The output of those quantizers with uniformstep or cell sizes may then be encoded using entropy encoders.

In an example embodiment, an error covariance matrix is calculated forthe spatially-transformed errors using an empirical moving averagecomputed over a window of time samples of the error signals. Theeigen-decomposition of the error covariance matrix is determined, andthe resulting eigen-vectors from the eigen-decomposition are used toform a KLT coefficient matrix. The KLT coefficient matrix may be sent tothe receiving node.

The filtering may performed using a matrix of predictive coefficients.In one example implementation, a matrix of predictive coefficients maybe estimated using empirical moving averages of (1) a cross-correlationof the multiple antenna signals and the quantized antenna signals and(2) an auto-correlation of the quantized antenna signals. In anotherexample, the matrix of predictive coefficients may be estimated usingrecursive empirical averages of (1) a cross-correlation of the multipleantenna signals and the quantized antenna signals and (2) anauto-correlation of the quantized antenna signals. In either case, thematrix of predictive coefficients may be sent to the receiving node.

Another aspect of the technology includes a decompression method. Acompressed signal that corresponds to a multi-antenna signal, themulti-antenna signal including information associated with a usercommunication received over multiple antennas, is received anddecompressed based on one or more correlations in space and in time toreconstruct a representation of the multi-antenna signal. Thereconstructed representation of the multi-antenna signal is thenprovided for further processing or output. The reconstructedrepresentation of the multi-antenna signal is complex-valued, sampled,and multi-dimensional.

Another aspect of the technology includes reconstructing estimates ofthe quantized errors from the quantized, spatially-transformed errors.The reconstructing step includes decoding the quantized,spatially-transformed errors from digital to analog form and applying aninverse spatial transform to the decoded errors to produce thereconstructed, quantized error estimates. The reconstructed, quantizederror estimates are combined with corresponding predictions of multipleantenna signals to produce quantized antenna signals, and the quantizedantenna signals are filtered in time and space to generate predictionsof multiple antenna signals. The coefficients associated with theinverse spatial transform may be received from the transmitting node.

In one example implementation, the compressed signal includes, for eachantenna signal, an error signal indicating an error between the antennasignal and a prediction of the antenna signal. The decompressingincludes converting the error signals from a digital format to an analogformat, applying an inverse spatial linear transform to the errors togenerate corresponding quantized error signals, and performing infiniteimpulse response filtering on the quantized error signals to generatethe reconstructed representations of the multiple antenna signals. Theinverse spatial linear transform may include fixed, predeterminedinverse transform coefficients corresponding to an inversediscrete-cosine transform (DCT), an inverse discrete Fourier transform(DFT), or an inverse discrete wavelet transform (DWT). Alternatively,the inverse spatial linear transform may include adaptively computedinverse transform coefficients which are received from a transmittingnode transmitting the compressed signal. For example, the inversespatial linear transform coefficients correspond to an inverseKahunen-Loeve transform (KLT).

The infinite impulse response filtering includes summing the errorsignals with corresponding predicted antenna signals to generatereconstructed representations of multiple antenna signals. Thereconstructed multiple antenna signals are filtered using a spatialtemporal prediction matrix of predictive coefficients to generate thepredicted multiple antenna signals. In one non-limiting exampleimplementation, the matrix of predictive coefficients is estimated basedon empirical moving averages of (1) a cross-correlation of the multipleantenna signals and the reconstructed antenna information and (2) anauto-correlation of the reconstructed representations of the antennasignals. In another non-limiting example implementation, the matrix ofpredictive coefficients is estimated based on recursive empiricalaverages of (1) a cross-correlation of the multiple antenna signals andthe reconstructed representations of the antenna signals and (2) anauto-correlation of the reconstructed representations of the antennasignals. In either case, the matrix of predictive coefficients may bereceived from the transmitting node.

Another aspect of the technology includes compression apparatus forcompressing information in signals received at multiple antennas.Processing circuitry is configured to process the received multipleantenna signals to decorrelate the received multiple antenna signalsover space and over time to generate a compressed signal. A transmitteris configured to transmit the compressed signal to a receiving node. Ina non-limiting example implementation, a predictor is configured togenerate predictions of the multiple antenna signals, and timedecorrelation circuitry configured to determine associated error signalsbetween the predicted multiple antenna signals and corresponding ones ofthe received multiple antenna signals to remove time correlation fromthe received multiple antenna signals and use the error signals togenerate the compressed signal. Space decorrelation circuitry isconfigured to transform the error signals using a linear spatialtransform into spatially-transformed errors to remove correlation inspace between the received multiple antenna signals. A quantizer isconfigured to quantize each of the spatially-transformed errors. Thecompressed signal includes the quantized, spatially-transformed errors.

Another aspect of the technology includes decompression apparatus havinga receiver configured to receive a compressed signal that corresponds toa multi-antenna signal, the multi-antenna signal including informationassociated with a user communication received over multiple antennasignal, a decompressor configured to decompress the compressed signalbased on one or more correlation in space and in time to reconstruct arepresentation of the multi-antenna signal, and an output configured toprovide the reconstructed representation of the multi-antenna signal forfurther processing or output. In a non-limiting example implementation,the compressed signal includes, for each of the antennas, an errorsignal indicating an error between the antenna signal and a predictionof the antenna signal. The decompression apparatus further includes ananalog-to-digital converter configured to convert the error signals froma digital format to an analog format, transform circuitry configured toapply an inverse spatial linear transform to the errors to generatecorresponding quantized error signals, and a filter configured toperform infinite impulse response filtering on the quantized errorsignals to generate reconstructed representations of the multipleantenna signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a non-limiting example of a multi-antenna radio nodecommunicating compressed multi-antenna signals with a receiver node;

FIG. 2 illustrates a Coordinated multi-point (CoMP) communicationsystem;

FIG. 3 is a flowchart diagram of non-limiting example compressionprocedures;

FIGS. 4A and 4B illustrate a non-limiting example diagram of multipleantenna signal compression apparatus;

FIG. 5 is a flowchart diagram of non-limiting example decompressionprocedures; and

FIG. 6 illustrates a non-limiting example diagram of multiple antennasignal decompression apparatus.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular nodes,functional entities, techniques, protocols, standards, etc. in order toprovide an understanding of the described technology. It will beapparent to one skilled in the art that other embodiments may bepracticed apart from the specific details disclosed below. In otherinstances, detailed descriptions of well-known methods, devices,techniques, etc. are omitted so as not to obscure the description withunnecessary detail. Individual function blocks are shown in the figures.Those skilled in the art will appreciate that the functions of thoseblocks may be implemented using individual hardware circuits, usingsoftware programs and data in conjunction with a suitably programmedmicroprocessor or general purpose computer, using applications specificintegrated circuitry (ASIC), and/or using one or more digital signalprocessors (DSPs). The software program instructions and data may bestored on computer-readable storage medium, and when the instructionsare executed by a computer or other suitable processor control, thecomputer or processor performs the functions.

Thus, for example, it will be appreciated by those skilled in the artthat diagrams herein can represent conceptual views of illustrativecircuitry or other functional units. Similarly, it will be appreciatedthat any flow charts, state transition diagrams, pseudocode, and thelike represent various processes which may be substantially representedin computer readable medium and so executed by a computer or processor,whether or not such computer or processor is explicitly shown.

The functions of the various illustrated elements may be providedthrough the use of hardware such as circuit hardware and/or hardwarecapable of executing software in the form of coded instructions storedon computer-readable medium. Thus, such functions and illustratedfunctional blocks are to be understood as being eitherhardware-implemented and/or computer-implemented, and thusmachine-implemented.

In terms of hardware implementation, the functional blocks may includeor encompass, without limitation, digital signal processor (DSP)hardware, reduced instruction set processor, hardware (e.g., digital oranalog) circuitry including but not limited to application specificintegrated circuit(s) (ASIC) and/or field programmable gate array(s)(FPGA(s)), and (where appropriate) state machines capable of performingsuch functions.

In terms of computer implementation, a computer is generally understoodto comprise one or more processors or one or more controllers, and theterms computer, processor, and controller may be employedinterchangeably. When provided by a computer, processor, or controller,the functions may be provided by a single dedicated computer orprocessor or controller, by a single shared computer or processor orcontroller, or by a plurality of individual computers or processors orcontrollers, some of which may be shared or distributed. Moreover, theterm “processor” or “controller” also refers to other hardware capableof performing such functions and/or executing software, such as theexample hardware recited above.

The technology described in this application includes an effective,low-complexity way to represent a complex-valued radio signal eitherreceived from or to be transmitted to a multiple antenna radio node,e.g., a base station. A spatial-temporal (ST) predictor compresses thedata associated with multiple antenna signals thereby reducing theirdynamic range. The spatial-temporal (ST) predictor exploits the factthat radio signals received from multiple antennas are often highlycorrelated in both space (i.e., across antennas) and time and uses asubstantially smaller number of bits to represent (quantize) a vectordifference signal between the predicted and the original antenna signalswhile maintaining the same level of incurred quantization distortion.Upon receipt of the quantized difference signal (i.e., the compressedsignal) sent over a backhaul channel by the multiple antenna radio node,a reproduction of the original multiple antenna signals may beconstructed (e.g., at a receiver) by filtering the difference signalusing a vector infinite impulse response (IIR) spatial-temporal filter.The filtering decompresses the received compressed signal. Thecoefficients associated with the spatial-temporal predictor can bepredetermined or determined in real-time based on the spatial andtemporal statistics of the multiple antenna radio signals. For thelatter case, the predictive coefficients may be sent (preferablyinfrequently) over the backhaul channel along with the quantized radiosignal in order to allow the multiple antenna radio signals to bereconstructed at the receiver. A low-complexity method of adaptivelycomputing the spatial-temporal (ST) predictor based on certaincorrelation matrix functions of the multiple antenna radio signals isalso described.

FIG. 1 illustrates a non-limiting example of a multi-antenna radio node10 communicating compressed multi-antenna signals with a receiver node12. The multi-antenna radio node 10 includes two or more antennas fortransmitting and/or receiving antenna signals. In particularembodiments, each antenna signal is or was transmitted with the sameinformation though transmission over the air interface distorts thatinformation in ways that are specific to each antenna's location.Collectively, the various antenna signals form a multi-antenna signal.The multi-antenna radio node 10 includes a compressor 15 for compressingthis multi-antenna signal before transmitting the compressedmulti-antenna signal by a transmitter 16 over a channel 13 to thereceiver node 12. The compressor 15 performs operations described belowthat employ coefficients that in one example embodiment are sent overthe channel 13 (shown as a dotted line) to the receiver node 12.Alternatively, those coefficients may be predetermined (fixed) or aredetermined in the receiver node 12 so as to avoid having to send themover the channel 13. The receiver node 12 includes a receiver 17 forreceiving the compressed multi-antenna signal sent over the channel andin one embodiment any transmitted coefficients. A decompressor 18decompresses these signals using inverse operations from those used inthe compressor 15. The decompressed (expanded) multi-antenna signal isthen further processed and/or output as indicated in block 19.Compressing the multi-antenna signal saves considerable bandwidth on thechannel 13.

One non-limiting example application of the radio node 10 and receivernode 12 is a coordinated multi-point (CoMP) communication system, anexample of which is shown in FIG. 2. Mobile radios 20 communicate overan air interface with one or more of the multiple base stations 22. Eachbase station 22 includes multiple antennas 24, a compressor 15, and atransmitter 16 (as described above for FIG. 1) and communicates with acentral processor 26 over a backhaul link 28. The central processor 26can be a radio network node like a radio network controller (RNC), acore network node like an MSC or an SGSN, or an independent node. Thecentral processor 26 includes a receiver 17, a decompressor 18, andfurther processing and/or output 19 as described above for FIG. 1.

FIG. 3 is a flowchart diagram of non-limiting example procedures thatmay be used by the radio node(s) 10. The compressor 15 receives multipleantenna signals that were transmitted with the same information (stepS1) and decorrelates those multiple antenna signals over space and timeto remove redundancies and generate a compressed signal (step S2). Incertain embodiments of radio node 10, the compressor 15 decorrelates themultiple antenna signals in time and space independently, with eitheroccurring first. In alternative embodiments, the compressor 15decorrelates the multiple antenna signals jointly in time and space. Ingeneral, the compressor 15 may decorrelate the multiple antenna signalswith respect to time and space jointly, independently, and/or in anyother appropriate manner. The transmitter 16 transmits the compressedsignal to the receiving node 12 or 26 over a channel 13 or 28 (step S3).

The operations of the compressor in accordance with one example detailedembodiment are now described. First the multiple antenna signals aremodels as follows: Let y[n]=[y₁[n], y₂[n], . . . , y_(n) _(α) [n]]^(T)denote an n_(α)-dimensional time-domain complex-valued, sampled,multiple antenna signal vector to be communicated through a backhaullink connecting a central processor from or to a particular basestation, where n_(α) denote the number of antennas at the base stationand nε{1, 2, . . . , N} denotes the sample time index. The temporal andspatial correlation (and thus redundancy) in the random process {y[n]}is represented using a vector auto-regressive (VAR) model given by:

$\begin{matrix}{{y\lbrack n\rbrack} = {{\sum\limits_{m = 1}^{M}{{A\lbrack m\rbrack}{y\lbrack {n - m} \rbrack}}} + {e\lbrack n\rbrack}}} & (1)\end{matrix}$where M is the model order, {e[n]} is an innovation process which ismodeled as a zero-mean, independent identically distributed (IID),vector Gaussian random process with R_(e)[m]=Ee[n]e[n−m]^(H)=Λ_(e)δ[m],δ[m] denotes the Kronecker-delta function, and e[n]≡[e₁[n], e₂[n], . . ., e_(n) _(α) [n]]^(T). The VAR model can match any power spectrum of themulti-dimensional radio signal with sufficiently large order M, and itleads to simple (low-complexity) compression methods that incurs littlelatency, as described below. Non-limiting example values for M might be2-8. But any suitable value for M may be used. Moreover, the VARcoefficients, as shown below, can also be computed efficiently based onmeasurements of the second-order statistics of the signal itself,enabling a low-complexity, adaptive implementation.

Based on the VAR model of the multi-antenna radio signal y[n] inequation (1), one approach might be to simply filter {y[n]} with avector FIR filter with a z-transform given by:

${H(z)} = ( {I - {\sum\limits_{m = 1}^{M}{A_{m}z^{- m}}}} )$in order to obtain the innovation process (approximated by an error ordifference) {e[n]}, which can then be quantized and sent over thebackhaul link. However, since the receiver node does not have access tothe original multiple antenna vector {y[n]}, as does the transmittingradio node, the encoding process is modified so as to integrate the FIRfiltering with the quantization of the innovation.

FIGS. 4A and 4B illustrate a non-limiting example diagram of multipleantenna signal compression apparatus that may be used to encode/compress{y[n]}. In general, the apparatus computes a predictive multiple antennavector signal ŷ[n] based on a quantized version y_(q)[n] of the originalmultiple antenna vector signal y[n], which is available at both thetransmitting and the receiving ends, as

${\hat{y}\lbrack n\rbrack} = {\sum\limits_{m = 1}^{M}{A_{m}{{y_{q}\lbrack {n - m} \rbrack}.}}}$

Since vector y[n] is often correlated in time, the error vector signalê[n]≡y[n]−ŷ[n], which serves as an estimate of the true innovation e[n],should have much smaller dynamic range than y[n] and can thus bequantized with fewer number of bits to achieve the same level ofquantization distortion. The quantized vector signal y_(q)[n] is simplygiven by the sum of the predictive vector signal ŷ[n] and the quantizedversion ê_(q)[n] of vector ê[n]. Sincey[n]−y _(q) [n]=y[n]−ŷ[n]−e _(q) [n]=e[n]−e _(q) [n],the fidelity of vector e_(q)[n] in representing vector ê[n] translatesdirectly into the fidelity of vector y_(q)[n] in representing thereceived, multiple antenna signals vector y[n]. The innovator 30 in FIG.4A includes n_(a) combiners 31 for determining a difference ê[n]provided to a linear spatial transform 32 and an error covariancecalculator 33.

The predictive vector signal ŷ[n] is provided by block 42 shown in FIG.4B, which in effect applies a vector infinite impulse response (IIR)filter 42 to the quantized error vector signal ê_(q)[n]. Morespecifically, block 42 generates the predictive vector signal ŷ[n] forthe next time instance by applying a vector finite-impulse-response(FIR) filter functioning as a spatial-temporal predictor 46 to the sumof the predictive vector signal ŷ[n] and the quantized error vectorsignal ê_(q)[n] from the previous time instances generated by the adder44. The quantized error vector signal ê_(q)[n] is generated by applyingan inverse spatial transform 40 shown in FIG. 4B to the output of thedecoders 38. The decoders 38 map the bits produced by theanalog-to-digital (A/D) encoders 36, e.g., through table lookups, to areconstructed or quantized version of the transformed error signal,which is then transformed to the quantized error vector signal ê_(q)[n]through the inverse spatial transform 40.

To minimize the dynamic range of the error vector signal ê[n], thepredictive matrix coefficients A≡[A₁, A₂, . . . , A_(M)] generated by apredictor coefficient calculator 48 shown in FIG. 4B may be computed byminimizing the variance of ê[n]:

$A = {{\underset{A = {\lbrack{A_{1},\mspace{11mu}\ldots\mspace{14mu},A_{M}}\rbrack}}{\arg\min}E{{\hat{e}\lbrack n\rbrack}}^{2}} = {\underset{A = {\lbrack{A_{1},\mspace{11mu}\ldots\mspace{14mu},A_{M}}\rbrack}}{\arg\min}E{{{{y\lbrack n\rbrack} - {\sum\limits_{m = 1}^{M}{A_{m}{y_{q}\lbrack n\rbrack}}}}}^{2}.}}}$

The orthogonality principle provides:

${{E( {{y\lbrack n\rbrack} - {\sum\limits_{m = 1}^{M}{A_{m}{y_{q}\lbrack {n - m} \rbrack}}}} )}{y_{q}\lbrack {n - k} \rbrack}^{H}} = 0$for all k=1, 2, . . . , M. In matrix form, this becomes:

$\begin{matrix}{{A\begin{bmatrix}{R_{y_{q}}\lbrack 0\rbrack} & {R_{y_{q}}\lbrack {- 1} \rbrack} & \ldots & {R_{y_{q}}\lbrack {M - 1} \rbrack} \\{R_{y_{q}}\lbrack 1\rbrack} & {R_{y_{q}}\lbrack 0\rbrack} & \ddots & \vdots \\\vdots & \ddots & {R_{y_{q}}\lbrack 0\rbrack} & {R_{y_{q}}\lbrack {- 1} \rbrack} \\{R_{y_{q}}\lbrack {M - 1} \rbrack} & \ldots & {R_{y_{q}}\lbrack 1\rbrack} & {R_{y_{q}}\lbrack 0\rbrack}\end{bmatrix}} = \mspace{419mu}{\lbrack {{R_{{yy}_{q}}\lbrack 1\rbrack}{R_{{yy}_{q}}\lbrack 2\rbrack}\mspace{14mu}\ldots\mspace{14mu}{R_{{yy}_{q}}\lbrack M\rbrack}} \rbrack\;\quad}} & (2)\end{matrix}$where R_(yy) _(q) [m]≡Ey[n]y_(q)[n−m]^(H) and R_(y) _(q)[m]≡Ey_(q)[n]y_(q)[n−m]^(H) are the multidimensional cross-correlationfunction of y[n] and y_(q)[n] and auto-correlation function of y_(q)[n],respectively. Equation (2) can be efficiently solved by a modifiedversion of the Whittle-Wiggins-Robinson (WWR) algorithm which computes Ain an order-recursive fashion, as summarized below. (The WWR algorithmsolves equation (2) when its right-hand side is [R_(y) _(q) [1] R_(y)_(q) [2] . . . R_(y) _(q) [M]] instead.)

Let A^((m))≡[A₁ ^((m)), A₂ ^((m)) . . . , A_(m) ^((m))] denote thesolution of equation (2) when M=m. In other words, A=A^((M)). Thefollowing algorithm solves equation (2) by recursively computing A^((m))until m reaches the desired order M. For notational simplicity, letR_(y) _(q) [1:m]≡[R_(y) _(q) [1], R_(y) _(q) [2], . . . , R_(y) _(q)[M]] and R_(yy) _(q) [1:m]≡[R_(yy) _(q) [1], R_(yy) _(q) [2], . . . ,R_(yy) _(q) [M]].

-   -   Step 1: Initialization (set m=1)    -   A₁ ⁽¹⁾=R_(yy) _(α) [1]R_(y) _(α) [0]⁻¹,    -   Ā₁ ⁽¹⁾=R_(y) _(α) [1]R_(y) _(α) [0]⁻¹, B ₁ ⁽¹⁾=R_(y) _(α[)        1]^(H)R_(y) _(α) [0]⁻¹,    -   Q₁=R_(y) _(α) [0]−Ā₁ ⁽¹⁾R_(y) _(q) [1]^(H) and S₁=R_(y) _(q)        [0]− B ₁ ⁽¹⁾R_(y) _(q) [1].    -   Step 2: Recursively compute the following quantities (until m        reaches M)

${\overset{\_}{P}}_{m} = {{R_{y_{q}}\lbrack {m + 1} \rbrack} - {\sum\limits_{i = 1}^{m}{{\overset{\_}{A}}_{i}^{(m)}{R_{y_{q}}\lbrack {m + 1 - i} \rbrack}}}}$$\begin{matrix}{{\overset{\_}{A}}_{i}^{({m + 1})} = {{\overset{\_}{A}}_{i}^{(m)} - {{\overset{\_}{P}}_{m}S_{m}^{- 1}{\overset{\_}{B}}_{m - i + 1}^{(m)}}}} & {{{{for}\mspace{14mu} i} = 1},2,\ldots\mspace{14mu},m}\end{matrix}$${\overset{\_}{A}}_{m + 1}^{({m + 1})} = {{\overset{\_}{P}}_{m}S_{m}^{- 1}}$$\begin{matrix}{{\overset{\_}{B}}_{m - i + 1}^{({m + 1})} = {{\overset{\_}{B}}_{m - i + 1}^{(m)} - {{\overset{\_}{P}}_{m}^{H}Q_{m}^{- 1}{\overset{\_}{A}}_{i}^{(m)}}}} & {{{for} = {i = 1}},2,\ldots\mspace{14mu},m}\end{matrix}$${\overset{\_}{B}}_{m + 1}^{({m + 1})} = {{\overset{\_}{P}}_{m}^{H}Q_{m}^{- 1}}$$Q_{m + 1} = {{Q_{m} - {{\overset{\_}{P}}_{m}S_{m}^{- 1}{\overset{\_}{P}}_{m}^{H}\mspace{14mu}{and}\mspace{14mu} S_{m + 1}}} = {S_{m} - {{\overset{\_}{P}}_{m}Q_{m}^{- 1}{\overset{\_}{P}}_{m}^{H}}}}$$P_{m} = {{R_{{yy}_{q}}\lbrack {m + 1} \rbrack} - {\sum\limits_{i = 1}^{m}{A_{i}^{(m)}{R_{{yy}_{q}}\lbrack {m + 1 - i} \rbrack}}}}$$A_{m + 1}^{({m + 1})} = {P_{m}( {{R_{y_{q}}\lbrack 0\rbrack} - {\sum\limits_{i = 1}^{m}{{\overset{\_}{A}}_{i}^{(m)}{R_{y_{q}}\lbrack i\rbrack}^{H}}}} )}^{- 1}$$\begin{matrix}{A_{i}^{({m + 1})} = {A_{i}^{(m)} - {{\overset{\sim}{A}}_{m + 1}^{({m + 1})}{\overset{\_}{A}}_{m + 1 - i}^{(m)}}}} & {{{{for}\mspace{14mu} i} = 1},2,\ldots\mspace{14mu},m}\end{matrix}$

-   -   Step 3: Finally, set A=A^((M)).    -   (Note that {Ā_(i) ^((m))}_(i=1) ^(m) and { B _(i) ^((m))}_(i=1)        ^(m) are auxiliary variables representing, respectively, the        forward and backward matrix prediction coefficients satisfying        similar (Yule-Walker) equations as (2) except that its        right-hand side becomes [R_(y) _(q) [1], . . . , R_(y) _(q) [m]]        instead of [R_(yy) _(q) [1], . . . , R_(yy) _(q) [m]].)

R_(yy) _(q) [m] and R_(y) _(q) [m] may be approximated by empiricalmoving averages {circumflex over (R)}_(yy) _(q) [n,m,N_(w)] and{circumflex over (R)}_(y) _(q) [n,m,N_(w)], respectively, which aregiven by:

$\begin{matrix}{{{\hat{R}}_{{yy}_{q}}\lbrack {n,m,N_{w}} \rbrack} \equiv {\frac{1}{N_{w}}{\sum\limits_{k = {n - N_{w} + 1}}^{n}{{y\lbrack k\rbrack}{y_{q}^{H}\lbrack {k - m} \rbrack}}}}} \\{= {\frac{1}{N_{w}}\begin{bmatrix}{{N_{w}{{\hat{R}}_{{yy}_{q}}\lbrack {{n - 1},m,N_{w}} \rbrack}} + {{y\lbrack n\rbrack}{y_{q}^{H}\lbrack {n - m} \rbrack}} -} \\{{y\lbrack {n - N_{w}} \rbrack}{y_{q}^{H}\lbrack {n - N_{w} - m} \rbrack}}\end{bmatrix}}}\end{matrix}$ $\begin{matrix}{{{\hat{R}}_{y_{q}}\lbrack {n,m,N_{w}} \rbrack} \equiv {\frac{1}{N_{w}}{\sum\limits_{k = {n - N_{w} + 1}}^{n}{{y_{q}\lbrack k\rbrack}{y_{q}^{H}\lbrack {k - m} \rbrack}}}}} \\{= {{\frac{1}{N_{w}}\begin{bmatrix}{{N_{w}{{\hat{R}}_{y_{q}}\lbrack {{n - 1},m,N_{w}} \rbrack}} + {{y_{q}\lbrack n\rbrack}{y_{q}^{H}\lbrack {n - m} \rbrack}} -} \\{{y_{q}\lbrack {n - N_{w}} \rbrack}{y_{q}^{H}\lbrack {n - N_{w} - m} \rbrack}}\end{bmatrix}}.}}\end{matrix}$for a correlation lag m=0, 1, . . . , M−1, where n denotes the currenttime index, and N denotes the window size. These moving averages can beupdated immediately as the latest sample y[n] and y_(q)[n] becomeavailable at the encoding end (the radio node 10). Alternatively, R_(yy)_(q) [m] and R_(y) _(q) [m] may be approximated by recursive empiricalaverages {circumflex over (R)}_(yy) _(q) [n,m;α] and {circumflex over(R)}_(y) _(q) [n,m;α], which are given by:{circumflex over (R)} _(yy) _(q) [n,m;α]≡(1−α){circumflex over (R)}_(yy) _(q) [n−1,m;α]+αy[n]y _(q) ^(H) [n−m]and{circumflex over (R)} _(y) _(q) [n,m;α]≡(1−α){circumflex over (R)} _(y)_(q) [n−1,m;α]+αy[n]y _(q) ^(H) [n−m]where αε(0,1) denotes a certain predefined forgetting factor, and{circumflex over (R)}_(yy) _(q) [0,m;α] and {circumflex over (R)}_(y)_(q) [0,m;α] are initialized to the all-zero matrix for all m. Thevectors R_(yy) _(q) [m] and R_(y) _(q) [m] are calculated in acorrelation computer 50 using y[n] and y_(q)[n], as shown in FIG. 4B,and are provided to the predictor coefficient calculator 48 which usesthem to generate the coefficient matrix A.

To reduce the frequency of sending overhead for the VAR coefficients A,the compressor may use these empirical averages to compute A only aftereach block of T samples. For example, all signal samples between time[kT,(k+1)T−1] will assume the same set of VAR coefficients A computed attime kT based on {circumflex over (R)}_(yy) _(q) [kT,m;α] and{circumflex over (R)}_(y) _(q) [kT,m;α], or alternatively {circumflexover (R)}_(yy) _(q) [kT,m;α] and {circumflex over (R)}_(y) _(q)[kT,m;α], for any period index k.

Similar to the actual innovation e[n] at each time n, its estimate ê[n]is also spatially-correlated (across the multiple antennas), andtherefore, direct independent quantization of each component ê_(i)[n],for i=1, 2, . . . , n_(α), of ê[n]≡[ê₁[n], ê₂[n], . . . , ê_(n) _(α)[n]]^(T), although possible, is not an efficient way of quantizing ê[n].To exploit the spatial correlation, a linear spatial transformation isperformed in block 32 on the error signal ê[n] using transformcoefficients U from transform calculator 35 so that the transformederror vector signal w[n]≡[w₁[n], w₂[n], . . . , w_(n) _(α) [n]]^(T) hasmost of its energy (at a lower amplitude) concentrated in a smallernumber K_(α) of matrix elements representing the error, whereK_(α)≦n_(α), and thus the rest of its element can be discarded withoutaffecting the fidelity of the reproduced signal.

The linear transformation 32 may be fixed and pre-computed as, forexample, the discrete-cosine transform (DCT), the Discrete FourierTransform (DFT) or a discrete wavelet transform (DWT). In this case,there is no need to send the transform coefficients U along with thequantized prediction error e_(q)[n] to the receiving node 12.

Alternatively, the transformation 32 can be computed using adaptivelycomputed matrix coefficients, e.g., using the Kahunen-Loeve Transform(KLT) for the prediction error process {ê[n]} througheigen-decomposition of its marginal covariance matrixΛ_(ê)=Eê[n]ê^(H)[n], which is given by Λ_(ê)=UDU^(H), where U is aunitary matrix with columns being the eigenvectors of Λ_(ê), and D is adiagonal matrix with diagonal elements {λ_(ê,i)}_(i=1) ^(N) ^(α) beingthe eigenvalues of Λ_(ê). The KLT transformation matrix is simply givenby U. If adaptively computed matrix coefficients are used, then thetransform calculator 35 may also compute the matrix coefficients of theinverse transform and send them to the receiving node. If a KLTtransformation matrix U is used, then the inverse transform matrixcoefficients are given by the Hermitian, or the conjugate transpose,denoted by U^(H), of the matrix U.

The eigenvalues {λ_(ê,i)}_(i=1) ^(N) ^(α) of Λ_(ê) represent thecorresponding variances of the transformed predicted errors, which canbe used by the compressor to decide which errors, if any, should bediscarded. Alternatively, it may be preferred to allocate differentnumber of bits b_(i) to each transformed error output w[n] according toits significance as indicated by its variance. If the variance of acomponent is too small relative to those of the other components, nobits may be allocated to quantize it, i.e., it is discarded.

FIG. 4A shows quantizers 36 (labeled as A/D encoders) which uses theallocated number of bits b_(i) to quantize their respective inputs w[n]based on information provided by bit allocator 37. The bit allocator 37determines those allocations using the eigenvalues {λ_(ê,i)}_(i=1) ^(N)^(α) of the error covariance matrix Λ_(ê) calculated by transformcomputer 35 from the error covariance matrix Λ_(ê). There are differentmethods to determine {b_(i)}_(i=1) ^(N) ^(α) depending on the type ofquantizer used. For fixed-rate quantization, one can compute{b_(i)}_(i=1) ^(N) ^(α) using a high-resolution approximation as, e.g.,

$b_{i} \cong {\frac{b_{total}}{n_{a}} + {\frac{1}{2}{\log( {\lambda_{e,i}/( {\prod\limits_{j = 1}^{n_{a}}\;\lambda_{e,i}} )^{1/n_{a}}} )}}}$where b_(total) denotes the total number of bits available to quantizeeach sample of w[n]. Alternatively, one can also allocate equal numberof bits to the first k components, where σ_(k) ²>β and k≦n_(α). In thiscase, β is the minimum energy that determines if an error in the vectorw[n] from spatial transform 32 should be neglected. After calculatingb[n], the compression apparatus transmits b[n] to receiving node 12 as acompressed multi-antenna signal. Although not shown in FIG. 4A, for thesake of simplicity, the compression apparatus may perform aparallel-to-serial conversion on b[n] or otherwise process b[n] in asuitable manner before transmitting the compressed multi-antenna signalto receiving node 12.

Alternatively, one can apply the Breiman, Friedman, Olshen, and Store(BFOS) algorithm to optimally allocate the bits for a given set ofcomponent codebooks {C_(i)}. See Riskin et al., “Optimal bit allocationvia the generalized BFOS algorithm,” IEEE Trans. Info. Thy., vol. 37,pp. 400-402, March 1991, incorporated herein by reference. Thisquantizer for each coefficient described by Riskin et al. is afixed-rate quantizer, i.e., it generates a fixed total number of bitsb_(total) at each time instance. But the quantizer for each coefficientcan also be a variable-rate quantizer. In this example, it is preferredto use a quantizer with a uniform step or cell size in combination withan entropy encoder, such as a Huffman encoder, a Ziv-Lempel encoder, oran arithmetic encoder, which are well known to those skilled in the art,to generate a variable total number of bits at each time instance. Seefor example chapter 9 in Gersho and Gray, Vector Quantization and SignalCompression, Kluwer Academic Publishers, 1992. The fidelity of thereproduced signal is controlled by the choice of the step or cell sizeinstead of the choice of the total number of bits b_(total).

The eigenvalues {λ_(ê,i)}_(i=1) ^(N) ^(α) may also be needed to scaleeach error in vector w[n] and to scale the reconstructed components atthe decoding end if standard (Gaussian) quantization codebooks designedfor probability distributions with unit variance are used.

The marginal covariance matrix Λ_(ê) can be approximated in the errorcovariance calculator 33 by an empirical moving average computed over awindow of time samples as:

$\begin{matrix}{\Lambda_{\hat{e}} \approx {{\hat{\Lambda}}_{\hat{e}}\lbrack {n,N_{w}} \rbrack} \equiv {\frac{1}{N_{w}}{\sum\limits_{i = {n - N_{w} + 1}}^{n}{{\hat{e}\lbrack i\rbrack}{{\hat{e}}^{H}\lbrack i\rbrack}}}}} \\{= {\frac{1}{N_{w}}{\sum\limits_{i = {n - N_{w} + 1}}^{n}{( {{y\lbrack i\rbrack} - {\hat{y}\lbrack i\rbrack}} )( {\lbrack i\rbrack - {\hat{y}\lbrack i\rbrack}} )^{H}}}}} \\{= {\frac{1}{N_{w}}\begin{bmatrix}\begin{matrix}{{( {N_{w} - 1} ){{\hat{\Lambda}}_{\hat{e}}\lbrack {{n - 1};{N_{w} - 1}} \rbrack}} +} \\{{( {{y\lbrack n\rbrack} - {\hat{y}\lbrack n\rbrack}} )( {{y\lbrack n\rbrack} - {\hat{y}\lbrack n\rbrack}} )^{H}} -}\end{matrix} \\{( {{y\lbrack N_{w} \rbrack} - {\hat{y}\lbrack N_{w} \rbrack}} )( {{y\lbrack N_{w} \rbrack} - {\hat{y}\lbrack N_{w} \rbrack}} )^{H}}\end{bmatrix}}}\end{matrix}$where N_(w) denotes the number of time samples within the window, oralternatively, by a recursive empirical average computed as:Λ_(ê)≈{circumflex over (Λ)}_(ê) [n;α]={circumflex over (Λ)} _(ê)[n−1;α]+(y[n]−ŷ[n])(y[n]−ŷ[n])^(H)−(y[n−1]−ŷ[n−1])(y[n−1]−ŷ[n−1])^(H)where αε(0,1) denotes a certain predefined forgetting factor, and{circumflex over (Λ)}_(ê)[0;α] is initialized to the all-zero matrix. Tominimize the frequency of sending U, thereby saving bandwidth on thebackhaul, {λ_(ê,i)}_(i=1) ^(K) ^(α) , and {b_(i)}_(i=1) ^(K) ^(α) (whereK_(α) is the number of errors in w[n] with non-zero number bitsallocated), the transform calculator 35 may use these empirical averagesto compute (U, {λ_(ê,i)}_(i=1) ^(N) ^(α) •, {b_(i)}_(i=1) ^(N) ^(α) )only after each block of T samples. For example, all signal samplesbetween time [kT, (k+1)T−1] may assume the same spatial transform U andeigenvalues {λ_(ê,i)}_(i=1) ^(N) ^(α) computed at time kT based on{circumflex over (Λ)}_(ê)[n,N_(w)] or alternatively {circumflex over(Λ)}_(ê)[n;α], for any period index k.

The receiving node 12 performs a decompression method to recoverrepresentations of the multiple antenna signals. FIG. 5 is a flowchartdiagram of non-limiting example decompression procedures. First, acompressed signal that corresponds to a multi-antenna signal is received(step S10). Next, the received signal is decompressed based on one ormore correlations in space and in time to reconstruct a representationof the multi-antenna signal (step S11). The correlation in space andtime may represent a correlation of multiple antenna signals in spaceand in time performed independently and in any order, or the correlationmay represent a joint correlation of the multiple antenna signals inspace and time. In general, receiving node 12 may utilize anyappropriate form of correlation with respect to time and space,including any suitable joint or independent correlation of the twovalues. The reconstructed representation of the multi-antenna signal isthen provided for further processing or output (step S12).

FIG. 6 illustrates a non-limiting example diagram of multiple antennasignal decompression apparatus using to generate a reconstruction vector{y_(q)[n]} of the multiple antenna signal vector y[n]. The receiver 18receives the compressed multi-antenna signal b[n] at respective decoders50 which generate corresponding analog signals based on respective bitallocations b₁, b₂, b_(n) _(α) that are either predetermined oradaptively selected. If they are adaptive selected, those bitallocations may be received from radio station over the channel. Aninverse spatial transform 52 performs the inverse spatial transformusing inverse coefficient matrix U⁻¹ generated by transform calculator35 and sent to the receiving node 12 over the channel or generated atthe receiving node. The inverse spatial transform generates thequantized version ê_(q)[n] of the error signal ê[n] which is input to avector IIR filter 54 which combines it in respective combiners 58 withcorresponding predictive vector signals ŷ[n] generated by aspatial-temporal predictor 56 using coefficient matrix A operating onreconstruction vector {y_(q)[n]} of the multiple antenna signal vectory[n]. This is equivalent to filtering ê_(q)[n] by vector IIR filter 54with a matrix z-transform H(z) given by generated by the predictorcoefficient calculator 48 in the radio node and sent to the receivingnode 12 over the channel or generated at the receiving node 12.

${H(z)} = {( {I - {\sum\limits_{m = 1}^{M}{A_{m}z^{- m}}}} )^{- 1}.}$The output of the vector IIR filter 54 is the reconstruction vector{y_(q)[n]} that represents the multi-antenna signal now decompressed.

Since the VAR coefficients A computed by the predictor coefficientcalculator 48 are minimum-phase (in the sense that the roots of thedeterminant of the matrix

$( {I - {\sum\limits_{m = 1}^{M}{A_{m}z^{- m}}}} )$are all inside the unit circle), the IIR filter response is stable.

In an example embodiment, the matrix predictive coefficients A arediagonal matrices, which means that in effect, the spatial temporalpredictors 46 and 56 do not exploit the spatial correlation but only thetemporal correlation of the received compressed multi-antenna signal.The spatial correlation is exploited only through transform coding onthe prediction errors. This embodiment reduces the amount of overheadneeded to describe the predictive coefficients A (which are scalars) atthe expense of some performance degradation. These scalar predictivecoefficients can also be further restricted to be identical acrossdifferent antennas, in which case, the measurement of second-orderstatistics may be averaged across antennas as well. The modified WWRAalgorithm reduces to the Levinson-Durbin algorithm in this case.

While the model order M of the predictor is assumed to be fixed andpredetermined, if desired, the adaptive selection of M may be integratedin the order-recursive computation of the predictive coefficients byincrementing the model order only when the resulting reduction in theprediction error variance is sufficiently substantial. In this case, theadaptively selected model order M may be sent to the receiving node.

If the underlying frame structure and timing of the backhaul signalingis known, performance may be improved by using different (smaller) modelorders at the start of each frame to avoid mixing potentially differentstatistics of adjacent frames.

There are multiple advantages provided by this technology including, forexample, providing an effective way to compress complex-valued radiosignals either received from or to be transmitted to a remote basestation with one or more antennas. Both spatial and temporalcorrelations in the multi-dimensional radio signals are exploitedthrough joint spatial-temporal linear prediction to significantly reducethe amount of data that must be transmitted over the backhaul tocommunicate the ultimate information to be delivered. This means thecapacity of the backhaul is significantly increased. Moreover, thetechnology is universal and has relatively low implementationcomplexity. There is no need to assume any particular time or frequencystructure in the radio signal, and hence, is applicable for example toall 2G, 3G, and 4G standardized signals. The technology provides forcontinuous operation with little additional latency to the radio signal.Moreover, using linear prediction to compress analog signals in multipledimensions (e.g., compressing a multi-antenna radio signal) provides anexcellent tradeoff in performance and complexity. Accordingly, thetechnology may become important in backhaul-signal codecs in the future.

Although various embodiments have been shown and described in detail,the claims are not limited to any particular embodiment or example. Noneof the above description should be read as implying that any particularelement, step, range, or function is essential such that it must beincluded in the claims scope. The scope of patented subject matter isdefined only by the claims. The extent of legal protection is defined bythe words recited in the allowed claims and their equivalents. Allstructural and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the technology described, for it to beencompassed by the present claims. No claim is intended to invokeparagraph 6 of 35 USC §112 unless the words “means for” or “step for”are used. Furthermore, no embodiment, feature, component, or step inthis specification is intended to be dedicated to the public regardlessof whether the embodiment, feature, component, or step is recited in theclaims.

The invention claimed is:
 1. A compression method for compressinginformation in radio signals received at multiple radio antennas,comprising the steps of: receiving, over a plurality of radio antennas,multiple radio antenna signals associated with a user communication;decorrelating the received radio antenna signals over space and overtime to generate a compressed signal, and transmitting the compressedsignal to a receiving node, wherein the multiple radio antenna signalsare complex-valued.
 2. The compression method in claim 1, whereindecorrelating the received radio antenna signals over space and overtime comprises decorrelating the received antenna signals over space andtime independently.
 3. The compression method in claim 1, whereindecorrelating the received radio antenna signals over space and overtime comprises jointly decorrelating the received radio antenna signalsover space and time.
 4. The compression method in claim 1, wherein themultiple radio antenna signals are part of a coordinated multi-pointcommunication, and wherein the transmitting is over one or more backhaulcommunications links.
 5. The compression method in claim 1, wherein thedecorrelating includes: generating predictions of the multiple radioantenna signals; determining associated error signals between thepredicted multiple radio antenna signals and corresponding ones of thereceived multiple radio antenna signals; and using the error signals togenerate the compressed signal.
 6. The compression method in claim 5,further comprising: transforming the error signals using a linearspatial transform into spatially-transformed errors, and quantizing eachof the spatially-transformed errors, wherein the compressed signalincludes the quantized, spatially-transformed errors.
 7. The compressionmethod in claim 6, wherein the spatial linear transform includes fixed,predetermined transform coefficients corresponding to a discrete-cosinetransform (DCT), a discrete Fourier transform (DFT), or a discretewavelet transform (DWT).
 8. The compression method in claim 6, whereinthe spatial linear transform includes adaptively computed transformcoefficients, and wherein the method further comprises transmitting theadaptively computed transform coefficients to the receiving node.
 9. Thecompression method in claim 6, wherein the spatial linear transformincludes transform coefficients corresponding to a Kahunen-Loevetransform (KLT).
 10. The compression method in claim 6, furthercomprising quantizing the spatially-transformed errors using apredetermined fixed number of bits.
 11. The compression method in claim6, wherein one of more of the spatially-transformed errors is notquantized.
 12. The compression method in claim 6, further comprising:quantizing spatially-transformed errors using an adaptively selectednumber of bits that depends on corresponding variances of thespatially-transformed errors over time; allocating a larger number ofbits for quantizing a spatially-transformed error with a largercorresponding variance; and allocating a smaller number of bits forquantizing a spatially-transformed error with a smaller correspondingvariance.
 13. The compression method in claim 12, further comprisingallocating bits according to a logarithm of the variances of thespatially transformed errors for quantizing the spatially-transformederrors.
 14. The compression method in claim 12, further comprising usinga BFOS algorithm to allocate bits for quantizing thespatially-transformed errors.
 15. The compression method in claim 6,further comprising quantizing the spatially-transformed errors usingvariable-rate quantizers.
 16. The compression method in claim 6, furthercomprising: quantizing spatially-transformed errors using quantizerswith uniform step or cell sizes; and encoding the output of saidquantizers with uniform step or cell sizes using entropy encoders. 17.The compression method in claim 9, further comprising calculating anerror covariance matrix for the spatially-transformed errors using anempirical moving average computed over a window of time samples of theerror signals; computing the eigen-decomposition of the said errorcovariance matrix; and using the resulting eigen-vectors from theeigen-decomposition to form a KLT coefficient matrix.
 18. Thecompression method in claim 17, further comprising transmitting the KLTcoefficient matrix to the receiving node.
 19. The compression method inclaim 6, further comprising: quantizing the spatially-transformed errorsusing an adaptively selected number of bits allocated tospatially-transformed errors, and transmitting the bit allocations forthe spatially-transformed errors to the receiving node.
 20. Thecompression method in claim 5, wherein the generating step includesfinite impulse response (FIR) filtering.
 21. The compression method inclaim 6, wherein the generating step includes: reconstructing estimatesof the quantized errors from the quantized, spatially-transformederrors; combining the reconstructed, quantized error estimates withcorresponding predictions of multiple radio antenna signals to producequantized antenna signals; filtering the quantized antenna signals intime and space to generate the predictions of multiple antenna signals.22. The compression method in claim 21, wherein the reconstructing stepincludes: decoding the quantized, spatially-transformed errors, andapplying an inverse spatial transform to the decoded errors to producethe reconstructed, quantized error estimates.
 23. The compression methodin claim 22, further comprising transmitting coefficients associatedwith the inverse spatial transform to the receiving node.
 24. Thecompression method in claim 21, wherein the filtering step includes:filtering the quantized antenna signals using a matrix of predictivecoefficients.
 25. The compression method in claim 24, furthercomprising: estimating the matrix of predictive coefficients usingempirical moving averages of (1) a cross-correlation of the multipleantenna signals and the quantized antenna signals and (2) anauto-correlation of the quantized antenna signals.
 26. The compressionmethod in claim 24, further comprising: estimating the matrix ofpredictive coefficients using recursive empirical averages of (1) across-correlation of the multiple antenna signals and the quantizedantenna signals and (2) an auto-correlation of the quantized antennasignals.
 27. The compression method in claim 26, further comprisingtransmitting the matrix of predictive coefficients to the receivingnode.
 28. The compression method in claim 27, further comprisingtransmitting the matrix of predictive coefficients to the receivingnode.
 29. Compression apparatus for compressing information in radiosignals received at multiple radio antennas, comprising: processingcircuitry configured to process the received multiple radio antennasignals to decorrelate the received multiple radio antenna signals overspace and over time to generate a compressed signal, and a transmitterconfigured to transmit the compressed signal to a receiving node,wherein the multiple radio antenna signals are complex-valued.
 30. Thecompression apparatus in claim 29, further comprising: a predictorconfigured to generate predictions of the multiple radio antennasignals; time decorrelation circuitry configured to determine associatederror signals between the predicted multiple radio antenna signals andcorresponding ones of the received multiple radio antenna signals toremove time correlation from the received multiple radio antenna signalsand use the error signals to generate the compressed signal; spacedecorrelation circuitry configured to transform the error signals usinga linear spatial transform into spatially-transformed errors to removecorrelation in space between the received multiple radio antennasignals; and a quantizer configured to quantize each of thespatially-transformed errors, wherein the compressed signal includes thequantized, spatially-transformed errors.
 31. The compression apparatusin claim 29, wherein the processing circuitry is configured todecorrelate the received antenna signals over space and timeindependently.
 32. The compression apparatus in claim 29, wherein theprocessing circuitry is configured to jointly decorrelate the receivedradio antenna signals over space and time.
 33. The compression apparatusin claim 29, wherein the multiple radio antenna signals are part of acoordinated multi-point communication, and wherein the transmitting isover one or more backhaul communications links.
 34. The compressionapparatus in claim 29, wherein the processing circuitry is configuredto: generate predictions of the multiple radio antenna signals;determine associated error signals between the predicted multiple radioantenna signals and corresponding ones of the received multiple radioantenna signals; and use the error signals to generate the compressedsignal.
 35. The compression apparatus in claim 29, wherein theprocessing circuitry is configured to: transform the error signals usinga linear spatial transform into spatially-transformed errors, andquantize each of the spatially-transformed errors, wherein thecompressed signal includes the quantized, spatially-transformed errors.36. The compression apparatus in claim 35, wherein the spatial lineartransform includes fixed, predetermined transform coefficientscorresponding to a discrete-cosine transform (DCT), a discrete Fouriertransform (DFT), or a discrete wavelet transform (DWT).
 37. Thecompression apparatus in claim 35, wherein the spatial linear transformincludes adaptively computed transform coefficients, and wherein thetransmitter is further configured to transmit the adaptively computedtransform coefficients to the receiving node.
 38. The compressionapparatus in claim 35, wherein the processing circuitry is configured toquantize the spatially-transformed errors using a predetermined fixednumber of bits.
 39. The compression apparatus in claim 35, wherein oneof more of the spatially-transformed errors is not quantized.
 40. Thecompression apparatus in claim 35, wherein the processing circuitry isconfigured to: quantize spatially-transformed errors using an adaptivelyselected number of bits that depends on corresponding variances of thespatially-transformed errors over time; allocate a larger number of bitsfor quantizing a spatially-transformed error with a larger correspondingvariance; and allocate a smaller number of bits for quantizing aspatially-transformed error with a smaller corresponding variance. 41.The compression apparatus in claim 35, wherein the processing circuitryincludes variable-rate quantizers configured to quantize thespatially-transformed errors.
 42. The compression apparatus in claim 35,wherein the processing circuitry is configured to: quantizespatially-transformed errors using quantizers with uniform step or cellsizes; and encode the output of said quantizers with uniform step orcell sizes using entropy encoders.
 43. The compression apparatus inclaim 42, wherein the processing circuitry is configured to: calculatean error covariance matrix for the spatially-transformed errors using anempirical moving average computed over a window of time samples of theerror signals; compute the eigen-decomposition of the said errorcovariance matrix; and use the resulting eigen-vectors from theeigen-decomposition to form a KLT coefficient matrix.
 44. Thecompression apparatus in claim 35, wherein the processing circuitry isconfigured to quantize the spatially-transformed errors using anadaptively selected number of bits allocated to spatially-transformederrors, and the transmitter is configured to transmit the bitallocations for the spatially-transformed errors to the receiving node.